Internal combustion engines are ever more commonly being equipped with supercharging for increasing power, in which the charge air required for the combustion process in the engine is compressed, as a result of which a greater mass of charge air can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean effective pressure can be increased.
In general, for supercharging, use is made of an exhaust-gas turbocharger in which a compressor and a turbine are arranged on the same shaft, with the hot exhaust-gas flow being supplied to the turbine, expanding in said turbine with a release of energy, and thereby setting the shaft in rotation. The energy supplied by the exhaust-gas flow to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. It is advantageous for a charge-air cooler to be provided in the intake line downstream of the compressor, by which charge-air cooler the compressed charge air is cooled before it enters the at least one cylinder. The cooler lowers the temperature and thereby increases the density of the charge air, such that the cooler also contributes to improved charging of the cylinders, that is to say to a greater air mass. In effect, compression by cooling takes place.
Supercharging is suitable for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. For the same vehicle boundary conditions, it is thus possible to shift the load collective toward higher loads, where the specific fuel consumption is lower. This is also referred to as downsizing.
Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine. With targeted configuration of the supercharging, it is also possible to obtain advantages with regard to exhaust-gas emissions. With suitable supercharging for example of a diesel engine, the nitrogen oxide emissions can be reduced without any losses in efficiency. The hydrocarbon emissions can be favorably influenced at the same time. The emissions of carbon dioxide, which correlate directly with fuel consumption, likewise decrease with falling fuel consumption.
To adhere to future limit values for pollutant emissions, however, further measures are necessary. Here, the focus of the development work is on inter alia the reduction of nitrogen oxide emissions, which are of high relevance in particular in diesel engines. Since the formation of nitrogen oxides requires not only an excess of air but rather also high temperatures, one concept for lowering the nitrogen oxide emissions consists in developing combustion processes with lower combustion temperatures.
Here, inter alia exhaust-gas recirculation (EGR), that is to say the recirculation of combustion gases from the outlet side to the inlet side, is expedient in achieving this aim, wherein it is possible for the nitrogen oxide emissions to be considerably reduced with increasing exhaust-gas recirculation rate. Here, the exhaust-gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mfresh air), where mEGR denotes the mass of recirculated exhaust gas and mfresh air denotes the supplied fresh air.
To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates are required which may be of the order of magnitude of xEGR≈60% to 70%.
When operating an internal combustion engine with exhaust-gas turbocharging and with the simultaneous use of exhaust-gas recirculation, such as is the case in the internal combustion engine according to the disclosure, a conflict may arise if the recirculated exhaust gas is extracted from the exhaust line upstream of the turbine by means of high-pressure EGR and is no longer available for driving the turbine.
In the event of an increase in the exhaust-gas recirculation rate, the exhaust-gas flow introduced into the turbine then simultaneously decreases. The reduced exhaust-gas mass flow through the turbine results in a lower turbine pressure ratio, as a result of which the charge pressure ratio likewise falls, which is equivalent to a smaller compressor mass flow. Aside from the decreasing charge pressure, additional problems may arise in the operation of the compressor with regard to the surge limit of the compressor. Disadvantages may also arise with regard to the pollutant emissions, for example with regard to the formation of soot in diesel engines during acceleration.
For this reason, concepts are required which—in particular in the part-load range—ensure adequately high charge pressures with simultaneously high exhaust-gas recirculation rates. One proposed solution is so-called low-pressure EGR.
In contrast to the abovementioned high-pressure EGR arrangement, in which exhaust gas is extracted from the exhaust line upstream of the turbine and introduced into the intake line downstream of the compressor, in the case of a low-pressure EGR arrangement exhaust gas which has already flowed through the turbine is recirculated to the inlet side. For this purpose, the low-pressure EGR arrangement comprises a recirculation line which branches off from the exhaust line downstream of the turbine and opens into the intake line upstream of the compressor.
An internal combustion engine which is supercharged by means of exhaust-gas turbocharging and which is equipped with a low-pressure EGR arrangement is also the subject matter of the present disclosure.
The exhaust gas which is recirculated via the low-pressure EGR arrangement to the inlet side is mixed with fresh air upstream of the compressor. The mixture of fresh air and recirculated exhaust gas produced in this way forms the charge air which is supplied to the compressor and compressed, wherein the compressed charge air is supplied to the at least one cylinder downstream of the compressor.
Here, it is not disadvantageous that exhaust gas is conducted through the compressor during the course of the low-pressure EGR, because in general exhaust gas is used which has been subjected to exhaust-gas aftertreatment, in particular in the particle filter, downstream of the turbine. There is therefore no risk of depositions in the compressor which change the geometry of the compressor, in particular the flow cross sections, and thereby impair the efficiency of the compressor.
In contrast, problems may arise downstream of the compressor if the compressed charge air is cooled again before it enters the cylinders. During the course of the cooling, liquids previously contained in the charge air still in gaseous form may be condensed out if the dew point temperature of a component of the gaseous charge-air flow, in particular water, is undershot. Owing to the conventionally low arrangement of the charge-air cooler, condensate may collect in the cooler, which condensate is then introduced in uncontrolled fashion, in particular in the form of shocks, into the intake system and may lead to a severe disruption to the operation of the internal combustion engine or to irreversible damage of components downstream of the cooler. This phenomenon takes on greater significance with increasing recirculation rate, because with the increase in the recirculated exhaust-gas quantity, the proportions of the individual exhaust-gas components in the charge air inevitably increase, in particular the proportion of the water contained in the exhaust gas. Therefore, the exhaust-gas quantity recirculated via the low-pressure EGR arrangement is limited in order to reduce the water quantity condensed out or to prevent condensing-out. Here, the high recirculation rates required for a considerable reduction of the nitrogen oxide emissions are achieved through the additional use of a high-pressure EGR arrangement, including the disadvantages associated therewith.
To improve the emissions characteristics of an internal combustion engine, it is necessary for the measures and systems applied or used for reducing the pollutant emissions to be controlled and/or regulated as effectively as possible, that is to say provided with high-quality control and/or regulation.
Since exhaust-gas recirculation serves primarily for the reduction of the nitrogen oxide emissions (NOx), use is often made of a NOx sensor, which is arranged in the exhaust-gas discharge system, for regulating the EGR valve, that is to say for adjusting the recirculation rate. If the untreated NOx emissions of the internal combustion engine are higher than a predefined setpoint value, the EGR valve is adjusted in the direction of the open position in order to increase the EGR rate in order to reduce the nitrogen oxide concentration CNox,exhaust in the exhaust gas.
The concept for regulating the EGR rate has numerous disadvantages. Firstly, the sensor used for detecting the nitrogen oxide concentration is a very expensive sensor, which entails costs for example three times higher than those for an oxygen sensor. Secondly, and an aspect regarded as being particularly critical with regard to the quality of the EGR regulation, the sensor is thermally highly loaded, and at high risk of contamination, owing to its arrangement in the exhaust-gas discharge system, that is to say in the hot exhaust gas which has not undergone aftertreatment. The high exhaust-gas temperatures may considerably shorten the service life of the sensor, lead to damage or destruction of the sensor and thereby lead to failure of the EGR regulation. Contamination of the sensor with soot particles and oil contained in the exhaust gas may have the result that the nitrogen oxide concentration detected by the sensor is afflicted with a significant measurement error, that is to say the sensor outputs too low a nitrogen oxide concentration.
The inventors herein have recognized the above the issues and provide a system to at least partly address them. In one embodiment, a supercharged internal combustion engine is provided. The engine comprises a cylinder, an intake line in an intake system, for supplying charge air to the cylinder, an exhaust line for discharging exhaust gases, an exhaust-gas turbocharger including a turbine arranged in the exhaust line and a compressor arranged in the intake line, an exhaust-gas recirculation arrangement including a recirculation line which branches off from the exhaust line downstream of the turbine and opens into the intake line upstream of the compressor, and a sensor for detecting the concentration Ci,intake of a component i of the charge air in the intake system provided downstream of the opening of the recirculation line into the intake line.
In the internal combustion engine according to the disclosure, a sensor is provided in the intake system and not in the exhaust-gas discharge system. This has numerous advantages. The charge air—even after the compression in the compressor—is at a considerably lower temperature than the hot exhaust gas, such that that the thermal loading of the sensor is significantly lower and there is no risk of damage to or destruction of the sensor as a result of overheating. In fact, the sensor is advantageously equipped with an electric heater by which it can be heated to a minimum operating temperature.
Furthermore, the sensor, on account of its arrangement in the intake system and the recirculation of aftertreated exhaust gas via the low-pressure EGR arrangement, is not at risk of being contaminated by soot particles contained in the exhaust gas or by oil contained in the exhaust gas.
The effects described above considerably improve the quality of the EGR regulation in relation to the previous systems. There is no risk of failure of the EGR regulation as a result of thermal overloading. There is likewise no risk of a concentration Ci,intake being detected erroneously as a result of a contaminated sensor.
According to the disclosure, in order that the sensor is impinged on by charge air which contains exhaust gas already recirculated via the low-pressure EGR arrangement in addition to the fresh air, and not exclusively by fresh air, the sensor is or should be arranged downstream of the opening of the recirculation line into the intake line.
The sensor serves to detect by measurement the concentration Ci,intake of a component i of the charge air in the intake system, which concentration can be taken into consideration in an equation for determining the proportion Fintake of the charge air fraction resulting from the combustion, and/or for determining the recirculation rate xEGR. Using the sensor, it is therefore possible for the recirculation rate xEGR of the low-pressure EGR to be adjusted, that is to say for a shut-off valve, which is preferably arranged in the recirculation line and which serves as a low-pressure EGR valve for adjusting the recirculation rate, to be actuated.
With certain assumptions, it is possible in this way to realize closed-loop control of the low-pressure EGR, for example if the exhaust-gas recirculation takes place exclusively via the low-pressure EGR arrangement. That is to say also if an additional high-pressure EGR arrangement is provided but is deactivated. Furthermore, in the abovementioned scenarios, the concentration Ci,intake detected by means of a sensor or the proportion Fintake may be used to determine the nitrogen oxide concentration CNOx,exhaust in the exhaust gas, that is to say the untreated emissions of nitrogen oxides NOx. Here, it is possible to dispense with an expensive NOx sensor, which is arranged in the exhaust-gas discharge system, for determining the nitrogen oxide concentration CNOx,exhaust in the exhaust gas and/or for regulating the EGR valve, that is to say for regulating the exhaust-gas quantity recirculated via the low-pressure EGR arrangement.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.